The present invention relates to a multi-beam scanning device which simultaneously emits a plurality of scanning beams for scanning a surface to be scanned.
Conventionally, a laser beam printer which utilizes a scanning laser beam has been known as an outputting device for a computer and the like. The laser beam printer emits a laser beam to form a spot of light on a surface to be scanned (i.e., a photoconductive surface). The laser beam is ON/OFF modulated in accordance with an image data as the laser beam scans the scanned surface.
Recently, a gradation image has been desired by a user, and a laser beam printer which is capable of controlling a laser diode to emit various levels of intensity of the laser beam has become available.
In such a type of the printer, the intensity of the laser beam is controlled in accordance with the tone density of each pixel. In order to form an image having a desired gradation, controlling of the intensity of the emitted laser beam is important. That is, the laser diode should be driven quickly and accurately in accordance with the image data which carries the gradation information for each pixel. In order to control the intensity of the laser beam, an APC (Automatic Power Control) system has been employed. The APC system is a system in which the intensity of the emitted laser beam is detected with use of a photo sensor and the detection result is fed back to a driver of the laser diode so that the intensity of the output laser diode is adjusted quickly. An example of such a control is disclosed in a U.S. patent application Ser. No. 08/680,649, teaching of which is incorporated herein by reference.
On the other hand, recently, in order to increase an imaging speed and/or resolution of the formed image, a printer which emits more than one laser beam at the same time to form a plurality of scanning line images has been developed. In such a printer, a plurality of laser diodes are provided and driven to emit a plurality of laser beams. In such a printer using a plurality of laser beams, it is possible to form a gradation image by controlling intensity of laser beams emitted by the respective laser diodes.
FIG. 1 is a block diagram showing a main part of a conventional laser scanning unit which is capable of forming a gradation image with use of two laser diodes driven simultaneously.
The conventional laser scanning unit 200 has two laser diodes 2 and 4. The laser beam emitted by the laser diode 2 is converted into a parallel beam by a collimator lens 8A, and then split by a beam splitter 12A into two beams L1 and L2. The beam L1 is used for scanning, and the beam L2 is used for the APC operation.
The beam L1 split by the beam splitter 12A is converged in a direction perpendicular to a plane of FIG. 1 (which direction is referred to as an auxiliary scanning direction hereafter) by a cylindrical lens 14, and directed to a polygonal mirror 16.
The polygonal mirror 16 is rotated in a direction indicated by arrow A under a control of a polygonal mirror motor control circuit 22. The polygonal mirror 16 has a plurality of reflection surfaces. As the polygonal mirror 16 rotates, the incident angle of the laser beam with respect to a reflecting surface of the polygonal mirror 16 varies and accordingly the reflected beam scans on the photoconductive surface of the photoconductive drum in a main scanning direction which is indicated by arrow B.
The beam L1 is reflected by a reflection surface of the polygonal mirror 16, passed through an f.theta. lens, and scans on a surface of a photoconductive drum DRM of a printer in a direction indicated by an arrow B (i.e., the main scanning direction). If the photoconductive surface of the drum DRM is uniformly charged, and the intensity of the scanning laser beam L1 is modulated in accordance with an image data, an electrostatic line image is formed as the main scanning is performed.
The laser beam emitted by the laser diode 4 is made into a parallel beam by a collimator lens 8B, split into a beam L3 for scanning, and a beam L4 for the APC operation. The beam L3 is, similarly to the beam L1, converged by the lens 14 in the auxiliary scanning direction, deflected by the polygonal mirror 16, and scans on the photoconductive surface of the photoconductive drum DRM through the f.theta. lens 18. The incident positions of the beams L1 and L3 on the photoconductive drum DRM are slightly apart in the auxiliary scanning direction. Accordingly, at each scanning operation, the laser beams L1 and L3 scan on the surface of the photoconductive drum DRM in the main scanning direction B. Therefore, two adjacent parallel line images respectively extending in the main scanning direction are formed.
The photoconductive drum DRM is made rotatable about an axis X which extends substantially parallel to the main scanning direction B. By rotating the photoconductive drum DRM while the main scanning operation is repeatedly performed, a two dimensional image is formed on the photoconductive surface of the photoconductive drum DRM. In the description hereafter, a scanning line on the downstream side with respect to the rotating direction of the photoconductive drum is referred to as an upper scanning line, and the scanning line on the upstream side with respect to the rotating direction of the photoconductive drum is referred to as a lower scanning line.
A mirror 26 is provided to reflect the laser beams L1 and L3 directed to a position where the laser beams L1 and L3 do not contribute to form a latent image on the photoconductive surface of the photoconductive drum DRM. The laser beams L1 and L3 are reflected by the mirror 26 and directed towards a light receiving portion of a write position detecting circuit 24. The write position detecting circuit 24 generates a pulse signal as the laser beams L1 and L3 are incident to the light receiving portion thereof, and outputs the generated pulse as a horizontal synchronizing signal HS. The horizontal synchronizing signal HS is used for reading an image data from an image memory and for modulating the laser beams L1 and L3 to form an latent image on the photoconductive surface of the photoconductive drum DRM.
The latent image formed on the surface of the photoconductive drum DRM is developed by applying toner to form a toner image, and then the toner image is transferred and fixed onto a recording sheet (not shown).
The laser beam L2 split by the beam splitter 12A is converged by a lens 54A, passed through an aperture 58A, and is incident to a photo diode 50A. Similarly, the laser beam L4 is converged by a lens 54B, passed through an aperture 58B, and is incident to a photo diode 50B.
The photo diodes 50A and 50B generate electrical current corresponding to the intensity of received light, i.e., the photo diode 50A generates current corresponding to the intensity of the laser beam L2, and the photo diode 50B generates current corresponding to the intensity of the laser beam L4.
The electrical current generated by the photo diode 50A is converted into a voltage value by an I-V converter 46A, and the electrical current generated by the photo diode 50B is converted into a voltage value by an I-V converter 46B.
The output voltages of the I-V converters 46A and 46B are applied to inverting input terminals of differential amplifiers 28 and 30, respectively.
To the non-inverting input terminals of the differential amplifiers 28 and 30, image signals S1 and S2 are applied. Each of the differential amplifiers 28 and 30 outputs a voltage which is proportional to a difference between voltages applied to the non-inverting input terminal and to the inverting input terminal. V-I converters 32 and 34 convert output voltages of the differential amplifiers 28 and 30 into electrical current, and charge or discharge hold condensers 36 and 38, respectively.
Charged voltages of the hold condensers 36 and 38 are respectively applied to V-I converters 40 and 42, which output electrical current proportional to the voltages applied thereto. The laser diodes 2 and 4 emit laser beams, each of which corresponds to the electrical current flowing therethrough. The emitted laser beams are split by the beam splitters 12A and 12B. The laser beams L2 and L4 are received by the photo diodes 50A and 50B, I-V converted by the I-V converters 46A and 46B, and the intensity of the laser beams are feed-back controlled (i.e., the APC operation is performed) as described above. With this APC operation, the intensity of the laser beams are adjusted quickly in accordance with the input image signals S1 and S2.
FIGS. 2(a)-2(d) show a timing chart illustrating the relationship between the horizontal synchronizing signal HS, the clock signal CLK, the image signals S1 and S2.
In response to the horizontal synchronizing signal HS, which is generated by the write position detection circuit 24, the clock signal CLK having a predetermined period T is generated. Then, synchronously with the clock signal CLK, the image data S1 and S2 are input to the differential amplifiers 28 and 30, respectively.
As described above, with a conventional laser scanning unit capable of emitting a plurality of laser beams at the same time, it is possible to form a gradation image. However, in order to perform the APC operation for each pixel, the number of the beam splitters, converging lenses, apertures and photo diodes should be the same as the number of the laser diodes. Therefore, employment of a plurality of laser diodes increases manufacturing cost, and further increases the size of the scanning device.